1. Field of the Invention
This invention relates to a sliding clasp fastener, more particularly to such a fastener which can be opened and closed in either of two reciprocal directions.
2. Prior Art
A variety of sliding clasp fasteners or zippers are known which are equipped with a pair of sliders, providing an option for the user to manipulate either of the two sliders to open or close the fastener from either direction. Such fasteners have coupling elements formed by press, injection or die-casting into a symmetrical mating structure, the element in a row on one stringer tape having a prong engageable with a groove in the corresponding element on the opposite tape. One problem posed by such form of coupling elements is that while the fastener is smoothly operative with the slider moving in one or the forward direction, the operation becomes sluggish or otherwise defective when the slider is moved in the other or the reverse direction account of the prong on one stringer getting jammed in the groove or severely striking against the front end wall of the groove on the mating stringer.
A means for coping with this problem is disclosed in Japanese Laid-Open Patent Publication 58-116946 wherein the coupling element is dented at an outer marginal edge of the groove so as to reduce interacting impact with the mating prong. However, such an element structure lacked, due to reduced contact area, an element-to-element coupling strength sufficient to withstand lateral pull or stresses applied perpendicularly to the plane of the fastener. Another difficulty was that the front end wall defining the groove is susceptible to deformation by repeated impinging contact with the corresponding prong, resulting in weakened interengagement of the fastener stringers.